On a sliding gate, the catcher post (also called the receive post or closing post) sits at the far end of the opening from the runout. It does three jobs. It stops the gate at the shut position. It holds the front edge so the gate does not kick sideways. And it usually carries the latch or lock.
A catcher post for a residential sliding gate is typically a 100x100x4 SHS in concreted footing. For commercial gates and longer leaves it goes up to 125x125x5 or 150x150x6 to handle the moment from a kicked or wind-loaded gate face.
Design-wise, the catcher post is also where the stop block, leading-edge latch, and any safety photo-eye get mounted, so it carries more than just the gate impact load.